I was wondering if you can change the css for a specific page (say the About section) with the custom upgrade?

Yes, you can change CSS just for the About page. CSS is very flexible. What you can do with it depends on the built in HTML code of the theme you are using. Most WordPress.com themes have body classes built in to every page so you can target each page or section separately.

To find out how to target a specific page on your site, view the page source in your browser and find the <body> tag. Look at the classes for that tag. Each post or page has an ID number and that is used in a specific tag for just that page. If you see a class like that, you can use it to target just that one particular page.

For example, I looked at the body classes in the page source for http://whatthepupisup.com/about/ and I see one of the classes is "page-id-2". You can use that class name to make CSS rules for just the About page. Here is a basic example to illustrate:

.page-id-2 p {
color: red;
}

It will turn paragraphs red only for the About page and not affect any other page because the "page-id-2" class is only set on the About page.